A robust optimization process and reliable solution are achieved when \(\mathcal {D}\)  \(\le \)  5 \(\%\) , a criterion usually fulfilled for strong events, whilst for moderately strong and weak events \(\mathcal {D}\)  can be about 8–12 \(\%\) . We emphasize that a solution can be obtained even in the case of \(\mathcal {D}\)  \(\sim \)  20–30 \(\%\) , but with considerably larger uncertainties. Usually, it is necessary to possess about 2(n-1) data (NM stations), \(n\)  is the number of unknowns in the model, in order to be able to unfold the model parameters [1]}, [2]}, [3]}. Thus, it is sufficient to retrieve information from 15–20 NMs, specifically those in a polar region, whilst the mid-latitude stations provide the boundary conditions. However, this number of stations is reasonable in case of not complicated PAD and unidirectional SEP flux, such as GLE \(\#\)  59 or GLE \(\#\)  70 [4]}, [5]}. In case of more complicated PADs and/or bi-directional SEP flux, e.g., GLE \(\#\) 69 or GLE \(\#\) 71 [6]}, [7]}, the amount of required information considerably increases, leading to about 30–35 NM records necessary to perform a reliable analysis.
